Generally, water pumps are mainly mechanical water pumps which are connected to crankshaft pulleys of engines by belts and are operated by the driving force (engine driving force) of the crankshafts. However, because mechanical water pumps are operated along with rotation of engines, engine cooling cannot be independently controlled under optimum conditions. In addition, mechanical water pumps may increase the engine load, thus reducing the fuel efficiency of a vehicle. Further, the peripheral structure of an engine is complicated, and the volume thereof is increased.
In an effort to overcome the problems of such mechanical water pumps, an electronic water pump was introduced. Unlike the mechanical water pumps, a power source of the electronic water pump is a battery, and variable control is possible because it is operated by an electric motor using electric power. Therefore, engine cooling can be controlled under optimum conditions. An engine load can be reduced, and the peripheral structure of the engine can be simplified, and the fuel efficiency of the vehicle can be enhanced.
Particularly, in the case of environment-friendly EVs (electric vehicles), they have no engine which functions to distribute power, unlike typical internal combustion engines. Thus, a water pump which is provided to cool a drive motor, an inverter, a converter, different kinds of electronic controllers, etc. must be operated by electricity. Further, among vehicles having typical internal combustion engines, in the case of vehicles using ISG (idle stop & go) systems which are provided to enhance fuel efficiency and reduce discharge of exhaust gas, because the operation of an engine is automatically interrupted when the vehicle stops, (for example, while waiting for signal) an auxiliary electronic water pump which circulates engine cooling water for the purpose of heating a passenger compartment is essential.
Furthermore, unlike mechanical water pumps, electronic water pumps include a control board to control an electric motor. Different kinds of electronic components are mounted on the control board to control rotation of a rotating body. A temperature at which the electronic components can be normally operated is 120° C. or less. However, a temperature of a peripheral portion of the engine where the electronic water pump is disposed is about 150° C.
Under such high-temperature conditions, heat dissipation of the control board of the electronic water pump and a stator of the motor is a very important factor in enhancing the performance.
Recently several techniques that pertain to such heat dissipation were proposed. In Patent document 1 (EP 1 503083 A1), referring to FIG. 1, a control board 20 is installed in a lower end of a motor casing 22, and cooling water passes through a BLDC (brushless DC) motor so that heat generated from the control board 20 can be dissipated.
However, in Patent document 1, the flow of cooling water which circulates through the BLDC motor is not smooth. If some of cooling water stagnates in the motor casing 22, the temperature of the control board 20 may rather increase.
In Patent document 2 (EP 1 361368 B1), referring to FIG. 2, a housing 20 that has a control board 22 therein is disposed at a side of a motor casing 14. A separate cooling water path B is formed between the housing 20 and the motor casing 14 to maximize heat dissipation performance of the control board 22.
However, in Patent document 2, because the separate cooling water path B must be formed for heat dissipation of the control board 22, the entire structure is complex, and there are difficulties in machining, assembly and installation. In addition, technical realization is limited because of difficulty in manufacturing.